Embodiments described herein relate to accurate determination of sound speed, or of other characteristics derivable from sound speed, for a fluid in a conduit or tube. Characteristics derivable from sound speed include characteristics such as: fluid density, composition, purity, and temperature; the concentration of component A in a binary fluid mixture of A and B; and total protein concentration in a fluid such as blood. The fluid of interest may be flowing or at rest. Fluid sound speed typically is determined by launching an ultrasonic signal that propagates through a fluid along a path of known length, receiving the signal after it has propagated, and calculating the sound speed based on the path length and the time elapsed between launching and receiving of the signal. The ultrasonic signal may be launched and received using one or more transducers.
Sound speed includes a real part represented by the symbol c, which is typically expressed in units of m/sec. Sound speed also includes an imaginary part that is related to energy absorption processes, and which is described by an attenuation coefficient that is typically expressed in units of dB/cm. Sound speed is influenced by composition, temperature, pressure, and other variables or characteristics of the fluid. In liquids, for example, c is inversely proportional to the square root of the product of compressibility and density; thus any factor that affects compressibility or density may change the sound speed. In gases, c is inversely proportional to the square root of the molecular weight of the gas. In gases, c is directly proportional to the square root of temperature and of gamma, where gamma is the ratio of specific heat at constant pressure to specific heat at constant volume.
Using known relationships between sound speed and other variables or characteristics, sound speed measurements can lead to determination of values for the other variables or characteristics. Sound speed measurements alone cannot solve a multivariable problem, however. Even for a simple fluid such as pure water, measuring sound speed yields a unique value for a variable such as temperature only if the temperature is within a certain range and if the pressure is known. Viewed in another way, one challenge in measuring sound speed is to eliminate interfering variables. Another challenge is to measure sound speed accurately; for example, to measure c with an error of less than 0.1 percent, or to measure the attenuation coefficient with an error of less than 1 percent.
Fluid sound speed is sometimes determined using contrapropagation; this technique is typically used in a situation where the fluid is flowing. Ultrasonic signals are launched in opposite directions along two paths that, in the absence of flow, would be congruent, i.e. in exactly the same position and of equal length. Sound speed may be determined as the average of the speed along the two paths. Contrapropagation systems often try to employ long axial paths, as described in U.S. Pat. No. 5,179,862 and U.S. Pat. No. 6,065,350. In the '862 and '350 systems, the transducers are positioned external to the tube wall, so the paths traverse one or more regions of tube wall in addition to traversing the fluid of interest, and the differing sound speeds in the wall and the fluid can complicate the accurate determination of fluid sound speed. The total path length in the '862 and '350 systems is very long, so the wall path length is a small fraction of the total path length, and in practice the error introduced by the wall path length is sometimes ignored in long axial path systems.
In some situations, a long axial path may be impractical. For example, the tube may be positioned within a confined space that provides access to only a small axial portion of the tube. In such situations, it may be advantageous to employ a path that is orthogonal or oblique to the tube wall. For a path that is orthogonal or oblique to the tube wall, with transducers positioned external to the tube wall, the wall path length may be a significant fraction of the total path length, leading to problems in accurately determining fluid path length and fluid sound speed. U.S. Pat. Nos. 3,731,532 and 4,397,194 employ externally-positioned transducers and paths that are oblique or orthogonal to the tube wall. In the '532 apparatus, fluid sound speed is measured along one fixed path orthogonal to the tube. In the '194 apparatus, measurement of the transit time in the fluid along one fixed path orthogonal to the tube wall is interpreted in terms of tube inside diameter, assuming a known fluid sound speed. The '532 apparatus and the '194 apparatus use oblique paths to measure flow effects on transit times, from which the flow velocity is determined.
Ultrasonic determination of sound speed may use differential paths, which are paths whose lengths differ. Differential paths may help to determine sound speed accurately; multiple determinations along fluid paths of differing lengths provide information about linearity and consistency of the determinations. For example, ultrasonic determinations of fluid properties may use a flowcell or spool that includes one or more transducers, the spool being inserted into a tube or conduit containing a fluid of interest, and the transducers may be positioned on the flowcell to launch and receive signals along differential paths.
It is possible to determine fluid sound speed by penetrating a tube wall with one or more ports and positioning one or more transducers in direct contact with the fluid. This technique removes the inaccuracy caused by wall path contributions, but the technique is invasive: the fluid may become contaminated, and the transducer in contact with the fluid may disturb the flow or may cause debris to accumulate. A less-invasive technique is to use a flowcell that has no ports. Installation of the flowcell requires, for example, provision of a flanged opening of length equal to the flowcell's length or cutting of the tube wall to install the flowcell. Junctions between gasketed mating flanges or between the flowcell and the tube wall may cause problems because of seemingly minor misalignment or diameter mismatch.
A noninvasive technique for determining fluid sound speed may be advantageous in some settings, such as where the fluid-containing tube is part of a biomedical device or a semiconductor processing system. In such settings, it may be important to prevent contamination of the fluid and to maintain sterility of the fluid. In addition, it may be important to avoid any irregularity in the inner surface of the tube wall that might encourage accumulation of debris or, in the case of blood processing equipment, blood clot formation. A noninvasive apparatus may be defined as a system in which there is no penetration of the tube wall during installation or use of the fluid sound speed measuring apparatus. The systems described in the '862 and '350 patents are noninvasive, but each uses a long axial path, as noted above.
There is a need for a fluid sound speed determination system or method that is noninvasive and that eliminates unknown contributions from portions of the system that are external to the fluid of interest.